A typical boiling water nuclear reactor has a reactor core comprised of a plurality of fuel bundles in side-by-side relation to one another. Coolant/moderator flows upwardly within the fuel bundles and about the fuel rods within the fuel bundles and is converted to steam to produce power.
In U.S. Pat. No. 5,112,570, there is illustrated a fuel bundle having a plurality of part-length fuel rods (PLR). These PLR's are supported on the lower tie plate of each bundle and extend upwardly toward the upper tie plate. The rods, however, terminate short of the upper tie plate and typically between a pair of spacers along the fuel bundle. Between the upper end of each PLR and the upper tie plate, there is defined in the upper two-phase region of the fuel bundle a vent volume. This vent volume preferentially receives vapor from the two phase mixture of liquid and vapor in the upper region of the fuel bundle during power producing operations. There are many advantages associated with the use of PLR's including the increased vapor fraction within the vent volume and the pressure drop reduction in the upper two phase region of the bundle. These advantages include increased stability from thermal hydraulic and nuclear instabilities.
It will be appreciated that the mechanical hardware associated with fuel rod spacers causes local reduction in the flow area available for the vapor and liquid flowing through the fuel bundle. This causes significant pressure drops to occur as the flow passes each spacer. By using PLR's, the associated flow blockage effects of one or more of the full-length fuel rods extending through these spacers above the PLR is substantially eliminated. That is, because of the absence of a fuel rod at a lattice location above one or more PLR's, additional flow area through the spacer is obtained with consequent reduction in pressure drop across such spacer. As a consequence, significant flow diversion occurs into the lower pressure drop paths or vent volumes above the upper ends of the PLR's. Increased vapor and liquid are therefore pumped from surrounding flow passages, i.e., the interstitial regions around the adjacent fuel rods, into these vent volumes.
The creation of vent volumes above PLR's, and flow diversions resulting therefrom, however, can cause some reduction in critical power performance in the fuel bundle. Additional water may accumulate in the vent volume region above the PLR and thus be shunted out of the vent volume without heat generating contact with the remainder of the full-length fuel rods. Separation devices have been utilized to drive the dense liquid or water out of the vent volumes in generally lateral directions onto the surfaces of and into the interstitial regions between the full-length fuel rods to improve heat transfer performance. Such separation devices have generally taken the form of swirlers disposed in the vent volumes. These swirlers create a helical flow pattern causing the dense liquid to be driven laterally outwardly of the vent volume by centrifugal force. Such separation devices have been located within the spacers and have extended therefrom above or below the spacers. However, the separation devices are typically connected to the spacers, at least in part closing off the opening through the spacer, preventing access to a part-length rod in the registering opening or openings below the closed opening(s) of the superposed spacer(s). This complicates bundle assembly because typically the separation devices are not separate entities which can be removed and then reinstalled into the bundle assembly at the assembly site or in the field. For example, in the case of a failed part-length rod underlying one or more superposed spacers containing separation devices, the part-length rod cannot be removed from the bundle without disassembly of the bundle.